Thermal spraying, which is one widely used method among known surface modification methods, involves forming a coating on a substrate through spraying, onto the substrate, of a thermal spray powder that is made of a material such as metal, ceramic, and cermet, by using a heat source, for instance a combustion flame or a plasma jet. The thermal spray powder is typically heated to a temperature equal to or higher than its melting point or softening point by the heat source. Therefore, the substrate may undergo thermal alteration or thermal deformation, depending on the material and shape of the substrate. Accordingly, it is not possible to form a coating on a substrate of any material and shape by ordinary thermal spraying. This is disadvantageous in that the material and shape of a substrate used are subject to limitation.
Low-temperature thermal spraying processes have received attention in recent years as a novel method for solving such a disadvantage of conventional thermal spraying. For instance, Patent Document 1 discloses that cold spraying is used in order to form a chromium-containing coating on the sliding surface of a piston ring. Also, Patent Document 2 discloses a powder for cold spraying that contains granulated and sintered cermet particles made of tungsten carbide and metal.
However, thick thermal spray coatings are not easy to obtain efficiently by low-temperature thermal spraying processes such as cold spraying, on account of the low process temperature that is involved. This behavior is more pronounced in powders for thermal spraying that are made of a cermet than in those that are made of metal.